1. Field of the Invention
The present invention relates to a semiconductor laser and a method for producing the same, especially relates to a surface-emitting laser and a method for producing the same.
2. Description of the Related Art
A vertical-cavity surface-emitting laser (hereinafter, referred as a surface-emitting laser) has recently attracted to a number of researchers for the following reasons. The surface-emitting laser includes a laser oscillator interposed between laser mirrors. The laser oscillator is formed in a direction perpendicular to a substrate, and the laser mirrors have semiconductor crystalline surfaces from which the laser beam is emitted. With such a configuration, a surface-emitting laser can emit a laser beam at a low threshold current. The laser beam is emitted from a large light emitting surface and has a narrow emitting angle. The semiconductor substrate is not required to be cleaved so as to form light emitting surfaces, thereby improving the production yield of the surface-emitting laser. Moreover, such a configuration makes it possible to perform an operation test for the surface-emitting laser at a unit of a wafer before being divided into chips. Furthermore, a two-dimensional laser array can be easily formed. Such attractive features of the surface-emitting laser are expected to solve some of the problems of conventional edge-emitting lasers.
In the accompanying drawings, FIGS. 8A to 8D show a method for producing a conventional surface-emitting laser 300 which can be produced, for example, according to a method described in IEEE Photonic Technology Letters, Vol. 3 (1991), pp. 9-11. As is shown in FIG. 8A, on an n-GaAs substrate 301, a bottom mirror 312, a spacer layer 304, an active layer 305, a spacer layer 306, and a top mirror 313 are epitaxially grown in this order. The bottom mirror 312 is constituted by alternately forming several tens of n-Al.sub.0.08 Ga.sub.0.92 As layers 302 and n-Al.sub.0.6 Ga.sub.0.4 As layers 303. The top mirror 313 is constituted by alternately forming several tens of p-Al.sub.0.08 Ga.sub.0.92 As layers 307 and p-Al.sub.0.6 Ga.sub.0.4 As layers 308. The spacer layer 304, the active layer 305, and the spacer layer 306 are made of n-Al.sub.0.35 Ga.sub.0.65 As, p-GaAs, and p-Al.sub.0.35 Ga.sub.0.65 As, respectively.
Next, as is shown in FIGS. 8B and 8C, the top mirror 313 and the spacer layer 306 are subject to an etching step using a mixed solution of potassium dichromate, hydrogen bromide, and acetic acid, and then the active layer 305 is subject to a side etching step using a mixed solution of Clorox and water so that the area of the active layer 305 becomes smaller than that of the spacer layer 306. Finally, as is shown in FIG. 8D, a polyimide film 309 is formed on the spacer layer 304 so as to expose the surface of the top mirror 313. Then, electrodes 310 and 311 made of AuGe are formed so as to electrically connect the substrate 301 and the top mirror 313.
Furthermore, an atomic layer bonding technique for bonding different substrates to each other, which is utilized in a semiconductor laser producing method, is proposed by Conference on lasers and electro-optics 1991 in technical digest pp. 330-333. Hereinafter, this technique will be described with reference to FIGS. 9A to 9C.
As is shown in FIG. 9A, on an InP substrate 401, an n-InGaAs layer 402, an n-InP layer 403, a u-InGaAsP layer (.lambda.g=1.3 .mu.m) 404, a u-InGaAsP layer (.lambda.g=1.5 .mu.m) 405, and a p-InP layer 406 are epitaxially grown in this order using a metal organic chemical vapor deposition (MOCVD) method.
Then, under the condition where the surface of a p-GaAs substrate 407 is in contact with the surface of the p-InP layer 406 as is shown in FIG. 9B, a heat treatment is performed under a hydrogen atmosphere at a temperature of 670.degree. C. for 30 minutes so that atoms on the surfaces of the GaAs substrate 407 and the InP layer 406 are rearranged and combined with each other. Thus, the GaAs substrate 407 and the InP layer 406 are bonded together. Finally, as is shown in FIG. 9C, the InP substrate 401 is removed using hydrochloric acid. Then, the next step for producing a laser element is performed.
In the surface-emitting laser, the bottom mirror and the top mirror function as current paths for supplying a current to the active layer. Therefore, the bottom mirror and the top mirror are required to have a large area so as to have a low resistance. 0n the other hand, in order to promote recombination of carriers and to improve the emitting efficacy, it is desirable for the active layer to have a small area so as to effectively confine the current.
However, according to the above-mentioned conventional technique, the top mirror is etched so as to have a taper, thereby making the top surface area thereof decreased in size. As a result, there arises a problem in that the resistance of the top mirror cannot be reduced so much. In addition, in the case where the top mirror has an appropriate top surface area, the bottom of the top mirror may be too large to integrate into a two-dimensional array of the surface-emitting lasers.